Roller cone bits are well known in the art for boring into earth formations. Such bits may have one or a plurality of cones, typically three, each of which rotates about its own axis as the bit body is rotated about its axis. Each cone has cutting elements on the exterior that gouge and scrape the borehole bottom. In known roller cone bits, cutting elements may generally classified as milled tooth, in which the cutting elements are formed from a solid workpiece, or insert type, which individual cutter inserts are press fit into mating holes formed in the cone of the bit body. In mining and tunnel boring operations, rolling cone cutters often have disk type cutting elements.
For harder formations, tungsten carbide inserts define teeth extending from the exterior surface of the cone. These inserts are press fit into mating holes in the cones. Each insert has a cylindrical base that fits with an interference fit into the cone body. A cutting tip, which may have various shapes, protrudes from the base of the insert.
As the drill bit is used to bore into an earth formation, the inserts are worn away until the bit must be extracted from the hole and a new bit inserted. Such bits typically last approximately 40 to 100 hours of drill time in a typical formation. The useful lifetime of the inserts, and therefore the bit, depends on a number of well known factors, including the hardness of the material from which the inserts are made, the hardness of the earth formation, the weight on bit, and other factors. In very abrasive formations the useful lifetime of the bit is often limited by the total amount of carbide material available for resisting wear, especially the amount of carbide in the gage cutting area, a factor which is not well appreciated in the art. Increasing the number of inserts has been tried as a solution to this limiting factor, but the inserts must be spaced apart by a minimum distance so that sufficient body material is left between inserts in order to provide adequate mechanical support for the inserts. Use of diamond enhanced inserts (DEI) has been another solution that is often used, but this is a very expensive solution.
Thus, there remains a need for a structure and a method of installing wear elements in a roller cone bit that increases the total amount of material available for resisting abrasion. The present invention is directed to this need in the art.